In vehicles which have stalled due to the low skid number between the driven wheels and the pavement, such as in snow or on a wet lawn or on poor surface, “rocking free of a vehicle” is the long known and proven method to get out from the stuck condition. For example, if a vehicle is stuck in a snowbank, the driver engages the clutch in a manual shift transmission; the vehicle rolls backward out over the lowest point of the snowbank, since no torque is delivered to the wheels any longer. When the driver again puts torque on the driving wheel, the vehicle again moves in the desired driving direction, so that the vehicle is accelerated in the driving direction through the low driving torque and the stored potential energy until it reaches the return point again. At this point, the driver takes the torque from the driving wheels by disengaging the clutch, so that the vehicle again moves against the driving direction. If this procedure is repeated several times, the vehicle is “rocked free” from this snowbank.
From U.S. Pat. No. 4,914,983, it is known for a vehicle with an automatic transmission to activate a mechanism after actuating a switch, in which multiple ND/NR shifting sequences are performed fully automatically in order to free the vehicle from the snowbank. Hereby it is also suggested to introduce semi-automatically alternating ND/NR shifts, in which the time of the engaging of the gear can be controlled directly by the driver via the accelerator.
To reverse the output torque in conventional automatic transmissions, one to be disengaged clutch, actuated by frictional connection, has to be emptied and a second empty clutch, which has to be engaged, has to be filled. Even when the emptying and filling processes of the shifting elements can be performed simultaneously, these generally take between 400 ms–800 ms. These times correspond to a design of the transmission or the transmission control for shifting actions on dry pavements with a maximum in shifting comfort and still acceptable shifting times.
Such long intervals of time or idle time are not suitable for rocking free a vehicle, since the driver does not calculate in or consider the normal delay times for putting the automatic transmissions into gear. As a consequence it may occur that the attempt to free the car by the rocking action is without success, since the new “motor force” is brought about too late. Even if the driver recognizes the idle times of the system and tries to avoid these by prematurely actuating the selector lever, the premature recognition of the optimal point in time for reversing the frictional connection proves to be very difficult.
The present invention is based on the objective to state a procedure for optimizing the rocking free of a vehicle with an automatic transmission, a multiple clutch gear or an automated shift gear, which avoids the disadvantages of the state of the art. In particular, the rocking free of the vehicle shall be supported by an optimization of the shifting times of the shifting elements; in addition, a clear recognition of the need for rocking free the vehicle shall be made possible.